FIG. 1 is a circuit diagram showing a conventional constant potential generating semiconductor device. The operation of this circuit will be described briefly. During a first operation mode while a low "L" (ground potential) level signal is supplied to an input terminal V1, a constant potential having an optional intermediate value between two power source potentials V.sub.CC and V.sub.SS, e.g., (1/2) V.sub.CC is outputted from an output terminal V.sub.out. During a second operation mode while a high "H" level signal is supplied to the input terminal Vl, the output terminal is fixed to the "L" level. During the first operation mode with the output constant potential (1/2) V.sub.CC, if the potential at the output terminal V.sub.out shifts from (1/2) V.sub.CC, the potential at the output terminal V.sub.out is regulated to (1/2) V.sub.CC by turning on and off output transistors N14 and P14.
In the circuit shown in FIG. 1, the N-channel (output) transistor N14 and P-channel (output) transistor P14 are connected serially between the power sources V.sub.CC and V.sub.SS and constitute an output circuit OC. The connection point between these transistors N14 and P14 is connected to the output terminal V.sub.out.
The gates of the two transistors N14 and P14 are connected via nodes 11 and 13 to two output terminals C2 and C4 of a reference potential output circuit ROC. This circuit ROC automatically controls the potential at the output terminal V.sub.out to (1/2) V.sub.CC during the first operation mode. The circuit ROC is constructed of a P-channel transistor P11, an N-channel transistor N11, a P-channel transistor P12 and an N-channel transistor N12, respectively connected between the power sources V.sub.CC and V.sub.SS. A connection point C2 (output terminal) between the transistors P11 and N11 is connected to the gate of the transistor N11. A connection point C3 between the transistors N11 and P12 is connected via a node 12 to the back gate (substrate) of the transistor P12. The gate of the transistor P12 is connected to a connection point C4 (output terminal) between the transistors P12 and N12. The conductance g.sub.m of the transistors P11 and N12 is set considerably smaller than that of the transistors N11 and P12, to set the potential of the node 11 to (V.sub.c3 +V.sub.thN11) and the potential of the node 13 to (V.sub.c3 -V.sub.thP12). The output potential V.sub.out can be set to an optional value in accordance with the potential V.sub.C3 at the connection point C3 which is determined by the ratio of the transistors P11 and N12. In the following, the potential V.sub.C3 is assumed to be set to (1/2) V.sub.CC. During the first operation mode, if the potential at the output terminal V.sub.out is (1/2) V.sub.CC, both the output transistors N14 and P14 are turning on. If the potential at the output terminal V.sub.out shifts from (1/2) V.sub.CC, one of the output transistors N14 and P14 turns on to regulate the output potential to (1/2) V.sub.CC. In order to prevent both the output transistors N14 and P14 from turning on at the same time, the threshold values V.sub.thP12 and V.sub.thP14 of the transistors p12 and P14 are set to satisfy .vertline.V.sub.thP12 .vertline.&lt;.vertline.V.sub.thP14 .vertline. to provide an output dead band. This is accomplished by connecting the back gate of the transistor P12 to the node 12 which is lower in potential than that of the node 11. It is also possible to broaden the dead band by setting the threshold values V.sub.thN11 and V.sub.thN14 of the transistors N11 and N14 to satisfy V.sub.thN14 &lt;V.sub.thN14. This is accomplished by making the channel length of the transistor N14 longer than that of the transistor N11. The dead band of an output therefore becomes V.sub.thd (=V.sub.thdP +V.sub.thdN }where V.sub.thdP =.vertline.V.sub.thP14 .vertline.-.vertline.V.sub.thP12 .vertline., and V.sub.thdN =V.sub.thN14 -V.sub.thN11.
Switching between the first and second operation modes is performed by a control circuit CC. An N-channel transistor N13 is connected between the node 11 and power source V.sub.SS, and a P-channel transistor P13 is connected between the node 13 and power source V.sub.CC. The switching transistors N13 and P13 are turned on and off at the same time. Namely, they are turned off during the first operation mode, and turned on during the second operation mode. To this end, the transistor N13 is connected via an inverter INV to a NOR gate NOR, and the transistor P13 is directly connected to the NOR gate NOR. One of the input terminals of the NOR gate NOR is connected to the input terminal V1, and the other input terminal is connected to the power source V.sub.SS. As will be described later, when the low "L" level (V.sub.SS) is supplied to the input terminal V1, the first operation mode runs, and when the high "H" level (V.sub.CC) is supplied, the second operation mode runs. In order to fix the output terminal V.sub.out to the low "L" level during the second operation mode, an N-channel transistor N15 is connected between the output terminal V.sub.out and power source V.sub.SS, the gate of the transistor N15 being connected to the input terminal V1.
The operation of the circuit shown in FIG. 1 will be described below.
When the input terminal V1 is set to the V.sub.SS level, the first operation mode starts. Specifically, the transistor N15 as well as the control transistors N13 and P13 turn off to disconnect the nodes 11 and 13 from the power sources V.sub.CC and V.sub.SS. The potentials of the nodes 11 and 13 in this condition are determined by an output from the reference potential output circuit ROC. More specifically, as described previously, the potential of the node 11 is set to [(1/2) V.sub.CC +V.sub.thN11 }, and the potential of the node 13 is set to [(1/2) V.sub.CC +V.sub.thdP ]. As a result, if the potential of the output terminal V.sub.out is within the dead band between [(1/2) V.sub.CC +V.sub.thdP ]and [(1/2) V.sub.CC +V.sub.thN11 ], both the output transistors N14 and P14 turn off. If the potential of the output terminal V.sub.out shifts beyond the dead band, one of the transistors N14 and P14 turns on to regulate the output potential within the dead band.
When the input terminal V1 is set to the V.sub.CC level, the second operation mode starts. Specifically, both the control transistors N13 and P13 turn on. Therefore, the node 11 takes the V.sub.SS level to turn off the transistor N14, and the node 13 takes the V.sub.CC level to turn off the transistor P14. As a result, the output terminal V.sub.out will not be influenced by the nodes 11 and 13. Since the transistor N15 has been turned on, the output terminal V.sub.out is fixed to the V.sub.SS level.
FIG. 2 is a circuit diagram of another conventional constant level generating semiconductor device In the circuit shown in FIG. 2, the bias potential of the back gate of the transistor P12 is supplied from the node 11 in order to narrow the dead band at the output terminal V.sub.out. The different points of the circuit shown in FIG. 2 from the circuit shown in FIG. 1 are as follows. A P-channel transistor P25 is additionally provided. An output of the inverter INV is connected to the gate of the P-channel transistor P25, the source of the transistor P25 being connected to the node 11, and the drain thereof being connected via a node 26 to the back gate of the P-channel transistor P12. With such a circuit arrangement, the bias potential level (potential level of the node 26) of the P-channel transistor P12 is raised to the potential level of the node 11. The potential of the node 11 is higher than that of the node 12 by a threshold value of the N-channel transistor N11. By raising the potential of the node 26, the threshold value of the P-channel transistor P12 rises and a difference of threshold values between the P-channel transistors P14 and P12 becomes small. In this manner, the dead band at the output terminal V.sub.out becomes narrower than the circuit shown in FIG. 1.
In the circuits shown in FIGS. 1 and 2, the nodes 15 and 11 are fixed to the ground level and the nodes 13 and 14 are fixed to the power source potential level, during the second operation mode. In this state, the bias node 12 of the P-channel transistor P12 enters a floating state because the N-channel transistor N11 and P-channel transistor P12 (P25) become non-conductive. The cross section of the transistor P12 is shown in FIG. 3. During the second operation mode, an N well 1 serving as the bias layer for the node 12 (26) enters a floating state, while a P.sup.+ drain D serving as the node 13 is supplied with the power source potential via the P-channel transistor P13. Assuming that the potential of the node 12 (26) is lower than that of the node 13, electric charges of the power source from the P-channel transistor P13 are injected into a P type layer 3 via the node 13 [P.sup.+ layer (drain D)]and node 12 (26) [N well]. Electric charges from the power source are always injected into the P type layer 3, increasing the standby current.
The P type layer 3 is set to a negative potential by a biasing circuit in most cases. However, if the amount of electric charges via the N well 1 becomes in excess of the biasing ability of the biasing circuit, an ordinary bias potential will not be supplied to the P type layer 3, resulting in operation errors of other devices formed within the P type layer 3.